Abstract

Have you ever blown across a bottle's top and made a pleasant, resonant sound? If so, have you wondered how that note is made exactly? A bottle is actually what is called a closed-end air column. Clarinets and some organ pipes are examples of musical instruments of this type. In this science project, you will use bottles to investigate how the length of a closed-end air column affects the pitch of the note that it makes. All you need are some bottles, water, a ruler, and a chromatic tuner.

Objective

Determine the relationship between musical note frequency and either fluid level or air-column height when producing notes by blowing over the top of a partially-filled bottle.

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Introduction

Some musical instruments produce sound from vibrating strings, others from vibrating reeds, and still others from resonating columns of air. In this science project, you will study a simple example of the latter type of instrument: a narrow-necked bottle partially filled with water.

How do musical instruments make the sounds that they do? All sound is made by vibrations. The vibrations push and pull on air molecules, changing the air pressure around them. The pushes cause a local compression of the air (increase in air pressure), and the pulls cause a local rarefaction of the air (decrease in air pressure). The compressions and rarefactions are rapidly transmitted through the air from the original source as a wave making sound and are commonly called a sound wave. In summary, sound itself is a wave, a pattern, of changing air pressures.

The top part of Figure 1, below, represents the compressions (darker areas, increases in pressure) and rarefactions (lighter areas, decreases in pressure) of a pure-tone (i.e., single-frequency) sound wave traveling in air. If you measured the changes in pressure and graphed the results, you could see how pressure changes over time, as shown in the bottom of Figure 1, below. The peaks in the graph correspond to the compressions and the troughs correspond to the rarefactions.

Figure 1. Illustration of a sound wave as compression and rarefaction of air, and as a graph of pressure vs. time. (Image credits: Henderson, 2004)

For a sound wave, the frequency of the wave is related to the perceived pitch or musical note of the sound. The higher the frequency, the higher the perceived pitch. Technically, the frequency of a wave describes how many cycles of the wave happen during a certain amount of time. This is measured in Hertz (Hz), which is in cycles per second. On average, the frequency range for human hearing is from 20 Hz at the low end to 20,000 Hz at the high end. Figure 2, below, shows examples of sound waves of two different frequencies. The period in the wave is the time that it takes for a single cycle of the wave to pass. For more information on how sound waves make certain frequencies and harmonics, check out the Science Buddies project idea Do-Re-Mi with Straws or the resources in the Bibliography section below.

Sound waves travel in a certain way in closed-ended air columns. Musical instruments that function as closed-end air columns are basically tubes that are open at one end but closed or covered at the other end. Clarinets, like the one shown in Figure 3, below, and some organ pipes are examples of closed-end air columns. The clarinet is a closed-end air column because the mouthpiece end is closed during playing due to the mouthpiece, reed, and player's mouth, while the other end remains open. Some instruments can be changed to work as a closed-end air column by covering the end on the opposite side of the mouthpiece with a mute.

Figure 3. When a person plays a clarinet, it functions as a closed-end air column. (Image credits: Tulane Public Relations, Wikimedia Commons, 2013)

When making music using a closed-end air column, the note that is made depends on the length of the air column. But what exactly is the relationship? In this science project, you will investigate how musical note frequency changes based on the height of the empty space when producing notes by blowing over the top of a partially-filled bottle.

Terms and Concepts

Sound

Sound waves

Pitch

Musical notes

Frequency

Hertz (Hz)

Closed-end air columns

Questions

How are sound waves made?

What is the difference between the wave of a low frequency and the wave of a high frequency?

What are some examples of closed-end air columns?

How is the length of an air column related to the frequency of sound it can make?

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Materials and Equipment

Clean glass or plastic bottles with narrow necks (at least 5). Use bottles that are different shapes and sizes so you have a variety to test.

Water

Electronic chromatic tuner, such as Korg CA-40. Chromatic tuners are widely available in music stores and online through suppliers such as
Amazon.com.

Metric ruler

Optional: piano or keyboard for comparing notes

Lab notebook

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Experimental Procedure

In your lab notebook, make a data table like Table 1, below. You will be making a data table for each bottle you test. Table 1 has been partly filled in as an example; you will be recording your own results in your data table. Pick one of your bottles to test and label the data table with a description of that bottle, such as its volume (in milliliters [mL]), and height and diameter (in centimeters [cm]).

Table 1. In your lab notebook, make a data table like this one. You will make a separate data table for each bottle you test. This table has been partly filled in as an example; be sure to fill in your data table with your own data.

Try blowing across the top of the bottle you selected to make a resonant sound. Do this by touching your lower lip to the edge of the bottle, pursing your upper lip, and blowing gently over the opening, as shown in Figure 4, below. When you get the airflow just right, you will hear a musical note as the air column in the open bottle resonates.

Note: If you cannot make a note using the bottle when it is empty, try using a different bottle for this science project.

Figure 4. Blow across the top of a bottle, as shown here, to make a musical note.

Use the chromatic tuner to see which musical note is sounding. Record the note and the frequency of the note (in Hertz [Hz]) in the data table in your lab notebook.

Be sure to also record the water level (in cm) and air level (the height of the empty space in the bottle, in cm) in the same row in your data table.

For example, if the bottle is 19.5 cm tall and is empty, the water level would be 0 cm and the air level would be 19.5 cm. Similarly, if the bottle is half-filled with water, the water level and air level would both be 9.75 cm.

Optional: You could use a piano or keyboard to compare the notes.

To figure out the exact frequencies of the different notes, you can use the Michigan Technological University resource in the Bibliography at the end of the Background section.

Add some water to the bottle. You can fill the bottle with a certain amount of water, such as by filling it half full or three-quarters full, or you could just add a little water at a time. Then repeat steps 2–3.

How does the pitch of the note change? Is it higher or lower than before?

If you want, you could try to figure out how much water you need to add (or remove) to get a half-step change in pitch (e.g., from C to C-sharp, or from A to A-flat).

Repeat step 4 at least 8 more times, testing what note the bottle makes when it is filled with different amounts of water. This means you should test at least 10 different water (and air) levels in the bottle. This will give you a good amount of data to analyze and draw conclusions from.

Repeat steps 1–5 with each of your different bottles.

After you are done testing all of your bottles, make line graphs of your results. On both graphs, make a line for each bottle you tested, and on the y-axis (the vertical axis) put the frequency of the notes (in Hz). On one graph, put the water level (in cm) on the x-axis (the horizontal axis). On the second graph, put the air level (in cm) on the x-axis.

Analyze your results and try to make some conclusions.

How is the pitch of the note produced related to the water level? How is it related to the air level?

Do you see any patterns in the relationship?

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Variations

Using a series of identical bottles with different amounts of water, can you produce a series of notes in a musical scale?

What happens if instead of blowing over the top of the bottle, you tap the bottle (below the waterline) with a wooden mallet? How does the note produced by tapping change with water level in the bottle? Can you explain how this works?

You can also make musical notes by rubbing the rim of a wine glass with a wet finger. With wine glasses, the note frequency also changes as the fluid level is increased, but in the opposite direction of the bottles used in this experiment. Obviously, the physics of musical wine glasses must be different! Find out more with the
Science Buddies project Singing Wine Glasses.

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Ask an Expert

The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot.

Related Links

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